Sure it's cold - but cold enough to kill pine beetles?

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ASPEN - There is little doubt that subzero temperatures in the Roaring Fork Valley and throughout Colorado this month are making life miserable for humans. There is a lot of doubt on whether the air has been frigid enough for long enough to take a toll on the mountain pine beetles devastating the state's forests.

"It's not helping them, that's for sure," said Jan Burke, a silviculturist or tree specialist with the White River National Forest Supervisor's Office in Glenwood Springs.

Temperatures in Aspen plunged to minus 8 degrees Saturday and then minus 9 both Sunday and Monday, according to DayWeather Inc. While cold, none were record lows. Lows for all three dates were set in 1989.

Typically the research says that an uncharacteristically cold stretch is necessary before the snowpack building to decimate populations of mountain pine beetles, Burke said. Scientists believe warmer weather has allowed the pests to proliferate, particularly during droughts in the mid-1990s and this decade.

The results are evident in forests lining Interstate 70 in Summit and Eagle counties and in the Winter Park area. Miles upon miles of mature evergreen trees turned the color of rust before the needles completely fell off and left a dismal, barren landscape.

The Forest Service estimates that the mountain pine beetle alone has wiped out 3.18 million acres of forest in Colorado since 1996, while bark beetles in general have devastated 6.6 million acres in the state. Wyoming also has been hit hard.

"In general, the infestation has wiped out mature trees on over 4 million acres," Burke said.

Pitkin County has been better off than other areas of Colorado because it has a greater diversity of tree species, according to Burke. Lodgepole pine has been particularly hard-hit by the beetles. Pitkin County doesn't have as large a concentration of that species as its neighboring counties to the east.

Burke said it is going to be difficult to tell if the cold snap played a big role in killing off the insects.

Barbara Bentz, research entomologist with the Forest Service Rocky Mountain Research Center in Logan, Utah, said mountain pine beetles have evolved to handle cold weather.

"They have the ability to cold-harden," she said.

Bentz has been part of a team that studied the beetle's ability to tolerate cold temperatures in a field and lab study in 1999. In 2007, she and a partner created a model to predict cold tolerance based on the 1999 field and lab data.

She said timing is everything with cold weather. If weather chills really fast in the fall, down to minus 10 degrees Celsius, then the beetles get caught before they prepare themselves. Burke said early low temperatures also affect beetles because the snowpack hasn't insulated the first 4 feet of a tree trunk, where larvae are burrowed in.

Bentz's research shows mountain pine beetles can survive sustained low temperatures of minus 35 degrees Celsius later in the winter.

After examining temperatures in the Roaring Fork Valley at automated snow measuring stations operated by the federal government, Bentz noted that there were several days in a row when temperatures dipped to the high minus 20s. That probably produces a higher probability of mortality, Bentz said, stressing that it is hard to determine at this point if temperatures have been low enough.

"Time will tell. Maybe this won't be so good for them," she said.

Burke said beetle populations have already "dropped off" considerably within the past two years, so that will add to the difficulty assessing if the current cold weather affected them. She said the wet year two years ago appeared connected to the death of beetles. She and a Forest Service entomologist checked several trees in the Burnt Mountain area of the Fryingpan Valley, where they found a surprising discovery.

"We didn't find a single living bark beetle," she said.

She suspects that it was connected to the high water content in the trees.

In addition, beetles are dying off in the White River National Forest because they have destroyed so many trees over so broad of an area. The infestation has been so widespread that there are few mature, living trees of the right species left for the beetles to attack.